The present invention concerns a magnetic sensor according to the preamble of claim 1 and a method for determining the positions of an activation magnet according to the preamble of claim 20.
U.S. Pat. No. 5,210,838 discloses a piston and cylinder arrangement with means for determining and confirming a piston position. The piston moves axially inside a cylinder so that a permanent magnet attached to the piston moves with the piston. The magnetic field of the permanent magnet is measured with a Hall-transistor and is used for determining the position of the piston. This arrangement includes means for determining the piston position and has the disadvantage that Hall-effect transistors cover only a small section of the piston's travel distance. To determine piston positions along the entire stroke of the piston, multiple Hall-effect transistors are needed along the path of the piston. For long cylinders, this means that a multitude of Hall-effect transistors are disadvantageously required. To determine the positions of the piston, the individual Hall-effect transistors must be precisely positioned, and each can only determine a single position.
German patent document DE 38 03 293 A1 discloses a magnetically activated, analog and electric path monitor that is useful for linear motions and which can be used for measuring objects of the most diverse type. This path monitor has two Hall-sensors that are spaced apart from each other a distance greater than the distance of the path that is to be monitored and which are arranged at a predetermined distance parallel to the movement axis of the permanent magnet. To obtain a linear signal that unambiguously determines the position of the piston, it is necessary that the spacing between the Hall-sensors exceeds the distance over which measurements are to be taken. An analog electronic output signal is generated from the difference between the absolute values of the Hall-voltages of the two Hall-sensors. Arranging the Hall-sensors at distances that exceed the length over which measurements are to be taken is disadvantageous because both Hall-sensors must then be mounted beyond the reciprocating travel path of the piston. This requires more space and makes mounting the Hall-sensors on the cylinder more difficult. Under some conditions it might even be necessary to mount the Hall-sensors beyond the cylinders. In addition, only the travel path is measured with an analog signal, and no switching signals are generated.